The present invention relates to a high frequency amplifier, and a technology effective if applied to a high frequency amplifier which is built in a wireless communication system such as a cellular phone and which is variably inputted with power (amplitude or/and current) of an input signal in particular from outside and capable of being shared between both a GMSK (Gaussian filtered Minimum Shift Keying) system and an EDGE (Enhanced Data rates for GSM Evolution) system.
According to investigations of the present inventors, the following technology is considered as to a cellular phone with a high frequency amplifier built therein.
For example, the cellular phone has built therein a high frequency amplifier formed by integrally moduling power amplifiers using semiconductor amplifying elements such as MOSFETs, GaAs-MESFET in an output unit on the transmitting side and a bias circuit therefor, etc. together with a baseband circuit having the function of controlling transmission/reception, etc., and a high frequency circuit having the function of modulating and demodulating a transmit/receive signal, etc.
In the high frequency amplifier for such a cellular phone, a control system for fixing power of a signal inputted to each of power amplifiers and varying a bias voltage for controlling the power amplifier is used as a power control method using a GSM (Global System for Mobile Communication). The control system for varying the bias voltage is a method of supplying a bias voltage to a gate terminal of each power amplifying transistor and controlling an output level so as to reach output power necessary to make a phone call.